Injection molding machines generally include an injection mechanism for forcing molten resin into a mold. Conventional hydraulic injection mechanisms include a hydraulic cylinder and an injection plunger. However, it is difficult to simplify the structure of hydraulic injection mechanisms and much labor is required to maintain the hydraulic cylinders and hydraulic fluid paths. Moreover, it is difficult to precisely control the reciprocation of the injection plunger.
To obviate these difficulties, injection mechanisms using motors as primary drivers have been also developed. A typical motor-driven injection mechanism includes a ball screw/ball nut mechanism having a rotatably mounted ball screw which is coupled to the output shaft of a servomotor, and a ball nut which is threadedly engaged therewith. The ball nut is mounted in such a manner that it is unrotatable relative to but movable in unison with an injection screw. The rotary motion of the motor output shaft is converted into linear motion of the injection screw through the ball screw/ball nut mechanism, so as to drive the screw in the direction of injection. However, motor-driven ball screw/ball nut injection mechanisms are expensive.
Moreover, in such motor-driven injection mechanisms, typically, the servomotor is operatively coupled to the ball screw through the medium of a power transmission which includes respective toothed pulleys fixed on the motor output shaft and the ball screw, and a timing belt stretched between such pulleys. In this connection, the power reduction ratio between the servomotor and the ball screw cannot be increased and in order to drive the injection screw with a desired driving force, in particular, with a great driving force, a servomotor which is high in output torque is required. However, since general-purpose servomotors having high output torque are not readily available, motor-driven injection mechanisms are high-priced.